Max c-notch hight wihout make a hole in the bed?

[kellegubben]

How high can i do the C-notch in the frame without have to make a hole or notch the bedline on my 1980 C10?
I want to go as low as possible without make any changes in the bedline.
The bed i have on my work fore some new paint jobb so its hard to mesure or try out the setup...

My rearend settup will be 4-link on Firestone F9000 bags. The axel is an 12-bolt and the rims will be 15".

Im hope someone here have done the same or know the answer!